The court wherein any tax judgment is entered may, at any time, upon satisfactory proof, vacate and set aside such judgment on the ground that the tax in question was paid before judgment was rendered, or that the land in question was not subject to taxation. Application to open such judgment may be summary, upon such notice to the purchaser and county auditor as the court may direct; and, if a defense is allowed to be interposed, the case shall proceed in all respects as in defended cases.